[System.IO.Compression] Fixed Zip entries with unset last write time to return the same as .NET.

.NET on Windows returns a fixed date for this, replicate the same behaviour in Mono.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=40916.

See also: https://github.com/NuGet/Home/issues/2518

3 files changed, 19 insertions, 0 deletions

diff --git a/mcs/class/System.IO.Compression/SharpCompress/Common/Zip/ZipEntry.cs b/mcs/class/System.IO.Compression/SharpCompress/Common/Zip/ZipEntry.cs
index 6ed742eeca3..2c0ec07e784 100644
--- a/mcs/class/System.IO.Compression/SharpCompress/Common/Zip/ZipEntry.cs
+++ b/mcs/class/System.IO.Compression/SharpCompress/Common/Zip/ZipEntry.cs
@@ -16,6 +16,13 @@ namespace SharpCompress.Common.Zip
                 this.filePart = filePart;
                 lastModifiedTime = Utility.DosDateToDateTime(filePart.Header.LastModifiedDate,
                                                              filePart.Header.LastModifiedTime);
+                if (lastModifiedTime == default(DateTime))
+                {
+                    // On .NET on Windows, for zip entries that don't have a last write time,
+                    // the return value for ZipArchiveEntry.LastWriteTime is:
+                    //   1/1/1980 12:00:00 AM, Ticks=624511296000000000
+                    lastModifiedTime = new DateTime(624511296000000000);
+                }
             }
         }
 
diff --git a/mcs/class/System.IO.Compression/Test/System.IO.Compression/ZipTest.cs b/mcs/class/System.IO.Compression/Test/System.IO.Compression/ZipTest.cs
index d461600299f..4205a1f40a0 100644
--- a/mcs/class/System.IO.Compression/Test/System.IO.Compression/ZipTest.cs
+++ b/mcs/class/System.IO.Compression/Test/System.IO.Compression/ZipTest.cs
@@ -264,6 +264,18 @@ namespace MonoTests.System.IO.Compression
 		}		
 
 		[Test]
+		public void ZipEnumerateArchiveDefaultLastWriteTime()
+		{
+			using (var archive = new ZipArchive(File.Open("test.nupkg", FileMode.Open),
+				ZipArchiveMode.Read))
+			{
+				var entry = archive.GetEntry("_rels/.rels");
+				Assert.AreEqual(new DateTime(624511296000000000).Ticks, entry.LastWriteTime.Ticks);
+				Assert.IsNotNull(entry);
+			}
+		}
+
+		[Test]
 		public void ZipEnumerateEntriesReadMode()
 		{
 			File.Copy("archive.zip", "test.zip", overwrite: true);
diff --git a/mcs/class/System.IO.Compression/test.nupkg b/mcs/class/System.IO.Compression/test.nupkg
new file mode 100644
index 00000000000..0a7fe0575c0
--- /dev/null
+++ b/mcs/class/System.IO.Compression/test.nupkg